Published February, 2015

Two equine researchers from the University of Kentucky's (UK) Maxwell H. Gluck Equine Research Center, in Lexington, presented their research on breeding and mare fertility during the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah. Here's a summary of their work:

Anti-Mullerian Hormone Helps Assess Aging Mares' Fertility

Equine researchers UK have been chasing anti-Mullerian hormone (AMH, which is produced by cells in fetal testes as well as granulosa cells) for quite some time. They've determined they can use it to test for cryptorchidism in male horses as well as ovarian tumors in mares. Now, they're trying to see if the hormone can predict an aging mare's remaining follicle count.

"Mare fertility declines with age in association with reduced follicle count," said study co-author Barry Ball, DVM, PhD, Dipl. ACT, Albert G. Clay Endowed Chair in Equine Reproduction at the Gluck Center. In other species, such as cattle and mice, "AMH is highly correlated with antral follicle count (AFC, or the total number of follicles that can be counted on ultrasound)."

He hypothesized that AMH concentrations might help predict an aging mare's AFC, reproductive longevity, and fertility.

In the study, Ball and his colleagues looked at the relationship between AMH, AFC, and follicle reserve in mares of different age groups. They examined 10 young (3-8 years), 16 middle-aged (9-18 years), and 19 older (over 18) mares using transrectal ultrasound and ELISA tests. In their results they found a strong relationship between AMH and AFC in older mares but not in young ones. They also discovered that mares with lower AMH concentrations had smaller follicles.

Additionally, Ball said they found that AMH concentrations in older mares that maintained pregnancies were significantly higher than in those that did not. "We're in the process of expanding on this," he added.

"Follicular reserve declines with age," Ball concluded. "Our goal is to be able to use AMH concentrations to examine reproductive longevity in older mares, but we need larger numbers to better understand how this can be applied clinically."

Estrogen Concentrations' Effects on Pregnancy Outcome

During pregnancy, mares experience elevated estrogen levels. Why? Well, that's largely unknown. What researchers do know is that when mares lose a pregnancy late in gestation, their estrogen levels are quite low. So a team of researchers lead by Ball recently tried to find out what the relationship is between estrogen concentrations and pregnancy loss.

In their study, the team treated six mares with 500 mg of letrozole, an estrogen inhibitor, every four days from Day 240 of gestation to foaling. Six untreated mares served as controls. They evaluated the mares' estrogen and androgen (a male sex hormone) concentrations weekly and other parameters such as fetal growth and the combined thickness of the uterus and placenta via ultrasound biweekly.

Ball said the treated mares' androgen levels increased and their estrogen levels dropped 90% immediately after the first treatment. All six mares gave birth to healthy foals with no abnormalities.

"Gestational length was unchanged, neonatal viability was normal, but birth weights of foals born to letrozole-treated mares were reduced by 15%," Ball said.

Based on these results, he concluded that reduced estrogen in late pregnancy does not affect pregnancy outcome. Supplementing mares in late gestation with estrogen, therefore, is likely unnecessary, he added.

"Reduced estrogen associated with pregnancy loss likely reflects disruption of placental function, but is not the cause of abortion," Ball said.

Does Uterine Edema Affect Pregnancy Outcome?

Edema, or fluid swelling, is often a sign that something's wrong. Pulmonary edema in horses' lungs, for instance, can be fatal. In certain eye structures it can lead to vision loss. In mares in estrus, however, some uterine edema in response to estrogen secretion is commonplace. But is it also benign?

Researchers from the Gluck Center and Rood & Riddle Equine Hospital recently studied uterine edema at ovulation to determine whether it affected pregnancy outcome. Ed Squires, PhD, Dipl. ACT (hon), professor at the Gluck Center, presented their results.

When examining a mare's uterus and ovaries during estrus, many veterinarians report seeing uterine edema as the mare approaches ovulation. The edema typically decreases just prior to ovulation. Some mares, however, continue to experience edema during or after ovulation, and no studies have been performed to determine whether this adversely affects fertility. Squires' team hypothesized that prolonged or excessive uterine edema would lower mares' pregnancy rates and increase incidences of early embryonic loss.

In the study, the team used ultrasound to examine 920 Thoroughbred mares for a total of 1,127 estrus cycles during the 2014 breeding season. Two days before ovulation as well as the day of, they recorded each mare's edema score on a scale of 0 to 4, with 0 indicating no edema and 4 being excessive, along with all other findings. They then classified the mares into three age groups—young (2-9 years), middle-aged (10-16 years), and old (17 and up)—and also noted their reproductive status (foaling, maiden, or barren) at the time of breeding.

Squires said they found that:

• Older mares had lower pregnancy rates (38%) than young (71%) or middle-aged (63%) mares;
• Pregnancy rates were similar among barren (66%), foaling (65%), and maiden (71%) mares;
• Barren mares older than 11 had lower pregnancy rates than younger barren mares;
• Barren mares had a greater incidence of early embryonic loss between Days 14 and 50 of gestation (13%) compared to foaling (8.7%) and maiden (5.5%) mares;
• Mares that had undergone a uterine lavage had a greater incidence (13%) of early embryonic loss than those not lavaged (7%). "This is probably an indication that mares that needed a lavage were likely those with fluid, older mares, or those with some other reproductive problem," Squires said;
• Mares with uterine fluid greater than 1 cm had an increased incidence of early embryonic loss (17%) compared with mares that had fluid less than 1 cm (8%). "This is probably because older mares have more fluid accumulation," Squires said;
• Only 6.5% of mares had excessive edema (scores 3 or 4);
• Higher edema grades correlated with greater amounts of uterine fluid; and
• Edema, antibiotic use, or other treatments did not significantly affect pregnancy rates.

"The fact that excessive edema did not lower pregnancy rates or increase embryonic losses was unexpected," Squires said. "Others have suggested that excessive edema at ovulation or after ovulation may be an indication of subclinical endometritis or perhaps an endocrine imbalance."

Mares' reproductive status and age's effect on pregnancy loss, however, was not surprising, as several previous studies had confirmed these correlations.

"In summary, maximum or excessive uterine edema near the time of ovulation did not adversely affect pregnancy rates or the incidence of early embryonic loss," Squires said, meaning that, based on this study's results, owners and veterinarians have little reason for concern when it comes to this type of swelling.

"Further studies are needed with more mares having grade 3 and 4 edema at ovulation," he added, "and data on the presence of bacteria in the uterus of mares with grade 3 and 4 edema would be useful in determining what is causing this excessive uterine edema."

Alexandra Beckstett is the managing editor at The Horse: Your Guide to Equine Health Care.